import java.util.Scanner;

public class Test {

    public static void main(String[] args) {
        //字符串找不同
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        String str1 = scan.nextLine();
        String str2 = scan.nextLine();
        int tmp = 0;
        for (int i = 0; i < str1.length(); i++) {
            tmp^=(int)str1.charAt(i);
        }
        for (int i = 0; i < str2.length(); i++) {
            tmp^=(int)str2.charAt(i);
        }
        System.out.println((char)tmp);
        scan.close();
    }
    public static void main3(String[] args) {
        //合并两个有序数组
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        int n = scan.nextInt();
        int m = scan.nextInt();
        int[] A = new int[n];
        int[] B = new int[n-m];
        for (int i = 0; i < m; i++) {
            A[i] = scan.nextInt();
        }
        for (int i = 0; i < n-m; i++) {
            B[i] = scan.nextInt();
        }
        int i = m-1;
        int j = n-m-1;
        int k = n-1;
        while(j >= 0 && i >= 0) {
            if(A[i] < B[j]) {
                A[k] = B[j];
                j--;
                k--;
            }else {
                A[k] = A[i];
                i--;
                k--;
            }
        }
        while(j>=0) {
            A[k] = B[j];
            j--;
            k--;
        }

       for (int x:A) {
           System.out.print(x+" ");
       }

        scan.close();
    }

    public static void main2(String[] args) {
        //反转字符串
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        String str = scan.nextLine();
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < str.length(); i++) {
            stringBuilder.append(str.charAt(i));
        }
        stringBuilder.reverse();
        System.out.println(stringBuilder);

        scan.close();
    }
    public static void main1(String[] args) {
        //压缩字符串
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        String str = scan.nextLine();
        StringBuilder stringBuilder = new StringBuilder();
        char[] arr = new char[str.length()];
        for (int i = 0; i < str.length(); i++) {
            arr[i]  = str.charAt(i);
        }
        int i = 0;
        int j = 1;
        while(j < str.length() && i < str.length()) {
            int count = 1;
            while(j < str.length() && arr[j] == arr[i]) {
                count++;
                j++;
            }
            stringBuilder.append(arr[i]);
            if(count > 1) {
                stringBuilder.append(count);
            }
            i = j;
            if(j+1 == str.length()) {
                stringBuilder.append(arr[j]);
            }
            j++;
        }
        if(stringBuilder.length() == str.length()) {
            System.out.println("NO");
        }else {
            System.out.println(stringBuilder);
        }

        scan.close();
    }
}
